This invention relates to an improved stepping motor for a timepiece, such as a quartz wristwatch. More particularly, it relates to an improved coil core and stator assembly for a wristwatch stepping motor.
Stepping motors for wristwatches are well known in the art and comprise a rotor with a permanent magnet, usually two pole, having a pinion meshing with a gear train and connected to drive the hands of the watch. The stepping motor is driven by periodic pulses, which in the case of a bipolar rotor are pulses of alternating polarity, furnished by an electronic circuit. Such a circuit is well known in the art and has a quartz-controlled oscillator, a dividing circuit for reducing the frequency, and a driving circuit which operates solid state switches controlling the application of drive pulses from a battery in the wristwatch.
Many arrangements have been suggested for reducing the thickness of a wristwatch movement while, at the same time, making the movement easy to assemble and employing the simplest parts in the movement. The arrangement, construction and assembly of the stepping motor is an important consideration in reducing the thickness of the timepiece movement. Every stepping motor requires a rotor, a stator cooperating with the rotor, a coil connected to the electronic circuit for developing magnetic flux, and a coil cure which is part of, or which is connected in magnetic circuit with the stator.
In order to simplify assembly, it has been proposed to make the coil core separate from the stator and to provide a coil wound on the coil core or placed on it as a prewound coil. The use of flat piece parts with minimum of machining is desirable to reduce the cost of the movement. A minimum number of parts which are easy to assembly is another aim. The separate parts need to be properly aligned and held securely in the frame. A review of the prior art follows which highlights the advantages achieved by the present invention.
U.S. Pat. No. 2,909,685--Szabo illustrates a single flat part comprising stator and coil core, but requiring winding the coil around the core, which is an expensive procedure.
U.S. Pat. No. 4,398,107--Fukushima and U.S. Pat. No. 4,361,772 illustrate a one-piece stator for a stepping motor attached to a separate core member by bolts, again requiring winding a coil on the core.
U.S. Pat. No. Re. 29,403--Yamazaki illustrate a stepping motor with coil and coil core overlapping especially machined areas on the stator piece and bolted thereto.
U.S. Pat. No. 4,297,838--Watanabe et al. disclose an effort to reduce the thickness of the timepiece by locating the coil core between the stator and the dial and providing a bent portion of the coil core to overlap part of the stator.
U.S. Pat. No. 4,647,218--Wuthrich assigned to applicant's assignee discloses a coil core member aligned with a stator member and having a bent portion lying on top of the stator. The added machining step of providing a bent or offset portion of the stepping motor core or stator is an extra manufacturing step which increases the cost of the movement.
U.S. Pat. No. 4,679,944--Sedlak et al issued July 14, 1987 describes a U-shaped combined stator and core member in which a prewound coil can be placed on the end of a core with a magnetic circuit completed by a separate piece overlapping the aforesaid member. Although this reduces the thickness of the stepping motor (except for the overlapping piece), the U-shaped member is expensive to manufacture.
Accordingly, one object of the present invention is to provide an improved stepping motor coil core and stator assembly which is easy to assemble and which provides a thin wristwatch movement.
Another object of the invention is to provide an improved stepping motor assembly which utilizes flat piece parts of inexpensive simple construction for the coil core and stator.
Another object of the invention is an improved stepping motor in which the parts are held in proper alignment in the frame.